With the release of Windows 10 Insider Preview Build 10565 to Windows Insiders in the Fast ring, Microsoft also announced that Windows 10 users who took advantahe of the free upgrade can now use their Windows 7, 8, or 8.1 product key to activate Windows 10 again when reinstalling the OS. When you clean install Windows 10, it is supposed to auto-activate, but if it doesn't for some reasons, then you can use your old product key.
We have received a lot of feedback from Insiders on making it easier to activate Windows 10 on devices that take advantage of the free upgrade offer to genuine Windows by using existing Windows 7, Windows 8 or Windows 8.1 product keys. If you install this build of the Windows 10 Insider Preview on a PC and it doesn’t automatically activate, you can enter the product key from Windows 7, Windows 8 or Windows 8.1 used to activate the prior Windows version on the same device to activate Windows 10 by going to Settings > Update & security > Activation and selecting Change Product Key. If you do a clean install of Windows 10 by booting off the media, you can also enter the product key from prior Windows versions on qualifying devices during setup. Refer to the Insider Hub for more information on these activation improvements including requirements.
Source: Windows